The present invention generally relates to radio frequency satellite communication systems and, more particularly, to a multi-beam and multi-band antenna system for communication satellites and for ground/aircraft terminals that communicate with multiple satellites.
Commercial as well as military communications have been evolving from single band systems to multi-band systems in order to achieve improved coverage, bandwidth, data throughput, and connectivity. The Defense Satellite Communications System (DSCS) systems use X-band (8 giga-Hertz (GHz)) while the Wideband Gapfiller Satellite (WGS) system being currently developed for U.S. Air Force uses X-band, K-band (20 GHz), and Ka-band (30 GHz) services. Future communication systems will be driven towards improved connectivity, anti-jamming performance, small terminal user support and increased data throughput. The Transformational Communications Architectures (TCA) studies are presently being conducted which may evolve into Transformational Communications Satellite/Asynchronous Protocol Specification (TSAT/APS) systems in the near future. These systems provide significantly increased communications capabilities to the existing EHF (45 GHz) satellites by adding the WGS services such that all three frequency bands K (20 GHz), Ka (30 GHz) and EHF (45 GHz) are simultaneously supported through a single antenna. In addition, for increased connectivity and flexibility TSAT systems are augmenting the multi-band services with multiple spot beams. Therefore, a single antenna system supporting multi-bands and multi-beams is required such that these beams provide a contiguous coverage over a theater area (region of the earth's surface) that can be reconfigured over the earth disk as seen by the satellite. Also, next generation Family of Advanced Beyond-line-of-sight Terminals (FAB-T) terminals for ground and aircraft are also required to support EHF and WGS services. These future communications requirements for satellite-based, ground-based and aircraft-based systems demand the development of multi-band and multi-beam antennas.
The existing antenna systems used for satellite payloads, aircraft terminals or ground terminals are designed to carry mostly single frequency band or, in some cases, dual frequency bands. These systems generally fall into one of the following three categories: (1) a single antenna supporting a single beam (either circular or shaped) at either a single frequency band or dual frequency bands; (2) a multiple aperture antenna system using three or four apertures, i.e., independent antennas, to produce multiple overlapping beams at a single frequency, such as disclosed by Sudhakar K. Rao, “Design and Analysis of Multiple-Beam Reflector Antennas”, IEEE Antennas and Propagation Magazine, Vol. 41, pp. 53-59, August 1999; and (3) a single antenna supporting dual or triple frequency bands and producing a single beam.
A single antenna system, however, that supports multiple frequency bands and multiple beams in each band simultaneously has not been observed in the prior art. The lack of such systems may be due, for example, to the fact that a single aperture sized for a low frequency band typically produces a much narrower beam at the high frequency band, especially when the bands are widely separated (e.g. more than one octave band of separation).
Gould, U.S. Pat. No. 6,208,312 B1, discloses an antenna that supports C and Ku band frequencies. The antenna employs a center-fed paraboloid with separate feeds for each band. Each feed covers a narrow bandwidth and the polarization is dual-linear.
Wong et al., U.S. Pat. No. 5,485,167, disclose a multi-frequency band, phased array antenna using multiple-layered, dipole arrays. In this design, each layer serves a distinct frequency band and all the layers are stacked together to form frequency selective surfaces. The highest frequency array is on the top of the radiating surface while the lowest frequency array is at the bottom-most layer. Disadvantages with this approach are the low antenna efficiency due to increased losses, interactions among layers, high mass, and high cost associated with phased arrays.
Zane Lo, U.S. Pat. No. 6,452,549 B1, discloses another version of a multiple-layered, multi-band antenna using printed dipole elements and slots. In this design, the low frequency layer is kept on top of the array while the high frequency layer is kept at the bottom side and both these layers share a common ground-plane at the bottom. It has disadvantages similar to those of Wong et al. described above.
Zhimong Ying et al., U.S. Pat. No. 5,977,928, disclose a multi-band antenna useful for radio communications (AM/FM) by using a multi-band swivel antenna assembly implemented in a coaxial medium. This approach works well over a narrow band but is not suitable at high frequencies. The antenna has very low gain due to its omni-directional radiation patterns.
Other approaches have employed dual-frequency antennas with frequency-selective surfaces (FSS) that are complicated, lossy, i.e., inefficient through energy loss, and work only for narrow band frequencies. An approach that avoids frequency-selective surfaces could provide significant advantages in efficiency, cost, and weight for providing multiple beams, and supporting multiple frequency bands.
As can be seen, there is a need for propagating radio frequency signals on multiple frequency bands and in multiple overlapping spot beams at each of the frequency bands. There is also a need for an antenna system that supports multiple frequency bands that are widely separated while also supporting multiple overlapping spot beams at each of the frequency bands. Furthermore, there is a need to provide for dual-circular polarizations for each beam and for each frequency band. Moreover, there is a need for an antenna system, with enhanced capabilities, that is applicable to next generation satellite payloads, aircraft antennas, and ground terminals.